This invention relates to steering systems for vehicles and, more particularly, to a rear wheel steering apparatus for a four-wheel steering system to achieve steering of the rear wheels in accordance with a steering angle of front wheels.
It is well known in the art to have a vehicle provided with a four-wheel steering system which includes a rear wheel steering apparatus designed to steer rear wheels in accordance with the steering of front wheels. In a typical example of the rear wheel steering apparatus, the rear wheels are steered in the same phase as the front wheels to improve the traveling stability at high cruising speed or/and in the reverse phase to steer the rear wheels in a direction opposite to the front wheels in order to achieve the maneuverability of the vehicle for smaller turns.
In general, the rear wheel steering apparatus is designed to have a steering shaft which is connected to a pair of rear wheels via a linkage mechanism in which the steering shaft is moved to one side of the rear wheels for thereby steering the rear wheels. The conventional rear wheel steering apparatus employs, for example, a system including a steering shaft in which threaded members are disposed in parallel to each other, and a pair of sliding members which are reciprocated on the threaded members to thereby move the steering shaft in left and right directions. In this system, one of the sliding members are coupled to the steering shaft by means of a clutch mechanism by which the reciprocating movement of the sliding members causes the steering shaft to move toward the sliding member. With this structure, the steering shaft is moved rightward and leftward in an axial direction and the rear wheels are steered in the rightward or leftward directions. Also, the rear wheel steering apparatus employs control means to enable setting of the target steering angle and the steering direction of the rear wheels in accordance with a steering information of the front wheels for thereby controlling the rear wheels in the steering direction by an amount equal to the deviation from the target steering angle. In addition, the rear wheel steering apparatus of the type described above is generally incorporated with a fail-safe system for automatically restoring the rear wheels to their straight cruising direction during abnormal operation of the system such as troubles in a steering angle sensor, etc. Consequently, when troubles occur in the system of the rear wheel steering apparatus, the vehicle which employs the four-wheel steering system is automatically switched over to a two-wheel steering mode for only front wheels.
For example, Japanese Patent Application No. 10-182090 which was filed by the same applicant discloses an electric rear wheel steering apparatus related to an invention of an electric steering control system. In order to move the afore-mentioned sliding members, the electric rear wheel steering apparatus includes the afore-mentioned threaded member (rotatable shaft) having a pair of threaded portions which are threaded from a center of the shaft in directions opposite to one another and which engage with a pair of sliding members (sliding block), respectively. Further, the electric rear wheel steering apparatus includes an electric motor to actuate the threaded members in their forward or reverse rotations for thereby moving the pair of sliding members in close to or away from one another due to reversed thread action. The electric rear wheel steering apparatus further includes a first motor driving circuit for normal use and a second motor driving circuit for use in trouble condition to supply electric current to the electric motor. The electric rear wheel steering apparatus also includes an electronic control unit arranged to produce an output signal during a normal cruising for supplying the electric current to the electric motor via the first motor driving circuit. The electric rear wheel steering apparatus further includes a fail-safe system having a trouble detecting circuit by which, when system troubles such as failures in the first motor driving circuit are detected, the second motor driving circuit supplies the electric current to the electric motor by which the sliding members are moved in close to each other toward their respective neutral positions.
However, the rear wheel steering apparatus mentioned above encounters a problem in that, when a control means such as the electronic control unit or the trouble detecting circuit is defected, the rear wheels can not be restored to the neutral position. Namely, the electric rear wheel steering apparatus can not render the motor driver circuit to be switched over from its normal operation mode to a trouble compensation mode even when the system trouble takes place. For this reason, the electric rear wheel steering apparatus functions to continue the steering operation of the rear wheels because of the electric current being applied to the electric motor from the motor driver circuit in response to the output signal produced by the electronic control unit even in a case where the system trouble takes place.
The present invention has been made in view of the above-described inadequacies of the related art and has an object to provide a new rear wheel steering apparatus which enables rear wheels of a vehicle to restore in a straight cruising mode even when a system trouble takes place in the control means.
The rear wheel steering apparatus which solves the above-mentioned drawbacks comprises an electric motor, a pair of threaded members which are threaded in directions opposite to one another, a pair of slide members movable in close to or apart from each other due to reversed thread action, and a steering shaft which steers the pair of rear wheels, provided that the electric motor rotates the threaded members so as to move the steering shaft connected to one of the slide members toward one of the rear wheels for thereby steering the rear wheels. The rear wheel steering apparatus further comprises control means for controlling the electric motor, and motor driver means comprised of a bridge circuit including four switching elements for driving the electric motor in its forward or reverse direction. The rear wheel steering apparatus features the provision of a trouble compensation circuit provided independently of the control means for turning on the switching elements for a given time interval, during a trouble condition taking place in the control means, so as to move the pair of sliding members in a direction close to each other when the control means is defected.
According to the rear wheel steering apparatus of the present invention, the trouble compensation circuit is arranged to turn on the switching elements for a given time interval by which the electric motor is driven for that time interval. In this manner, the rear wheel steering apparatus rotates the threaded members in a direction to approach each other due to rotation of the electric motor. As a result, the rear wheel steering apparatus causes the sliding members to return to their neutral positions during the given time interval for thereby restoring the steering shaft to its neutral position so that the rear wheels are restored to a straight cruising direction. In the meantime, the given time interval is designed to have a value sufficient for driving the electric motor so as to move the pair of sliding members in a direction to approach each other to enable the steering shaft to restore its neutral position even in a case where the steering shaft is moved for maximum distance. In this connection, the given time interval is set to a value of 10 seconds in the preferred embodiment of the present invention.
Other and further features, advantages, and benefits of the present invention will become more apparent in the following description taken in conjunction with the following drawings. It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings which are incorporated in and constitute a part of the invention, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention in general terms.